In the production of rigid plastic building panels such as siding and roofing panels it is frequently desirable to reproduce on the face of the panel the texture of wooden shake shingles or siding or other desired configurations. A commonly used technique for producing such panels is known as thermoforming. In a conventional thermoforming process a flat sheet stock is extruded and is then processed in either web form or as individual pieces. In the thermoforming process the plastic sheet stock is heated to a pliable state in a clamping frame and placed in contact with any of several types of molding apparatus. The molding apparatus imparts the desired configuration to the central areas of the sheet, after which the excess material is trimmed from the edges. Unfortunately, thermoforming places very definite limits on the configurations which may be used to provide for instance interlocking edge portions, nailing flanges, etc. on the panels. Typical molded rigid plastic material produced by thermoforming is described for instance in U.S. Pat. No. 3,593,479 to Hinds, the disclosure of which is incorporated herein by reference. Material produced in accordance with the teachings of the Hinds patent can have the desired texture on the face of the thermoformed portion; however, even utilizing the Hinds invention, the possibilities for forming various shapes of interlocked portions are considerably limited. Further, the interlocks which can be produced by thermoforming are generally not considered satisfactory because the amount of mold undercutting which can be used to form an interlock via thermoforming is limited by considerations of extracting the finished part from the mold and excessive thinning of material in the interlock area when a relatively small surface area of plastic must be conformed to a relatively large surface area of mold. It is therefore, an object of the invention to provide an improved plastic building panel having a thermoformed face but not subject to the disadvantages mentioned above.